Stephanie Yao/The OregonianPortland stores its treated drinking water at five historic open reservoirs, including this one at Mount Tabor Park. Health officials say the open-air reservoirs are risky, but reservoir supporters say there's little proof of that.

For the administrator of the Portland Water Bureau, the decision Wednesday to drain 7.8 million gallons of drinking water from a Mount Tabor reservoir comes down to six words:

From a gross-out perspective, that's enough to make residents wary of turning on the tap.

"I think I'm going to have a Coke with my lunch today," said city Commissioner Randy Leonard, who oversees the Water Bureau. The bureau recently began work on an $80 million project elsewhere to comply with federal rules to cover city reservoirs.

But does tossing out so much water -- at a cost of more than $36,000 -- make sense from a scientific or health angle?

"It's inappropriate behavior. But how many animals are doing that or birds?" he said. "I don't want to second-guess the city, but I can't think of anything chemically that would have me be concerned."

But Chris Wanner, director of operations for the Water Bureau, said he was worried -- though less about the urine than about unknown items thrown into the water by a group of five people who hung around Mount Tabor early Wednesday.

Even so, Portland's five open reservoirs are susceptible to dead rodents, birds or bird poop. Water quality is tested, but reservoirs aren't drained because of dead animals.

"We look at that as part of the business of open reservoirs," Wanner said.

Water officials disconnected the same reservoir in April 2010 after tests found E. coli in samples. Five months before that, they issued a rare boil alert after tests at a Washington Park reservoir found E. coli.

Dr. Gary Oxman -- the Multnomah County health officer who advises officials on infectious diseases such as salmonella, hepatitis and H1N1 -- said the average bladder holds 6 to 8 ounces of urine and would be vastly diluted in a reservoir.

"The health risk associated with that is really, really tiny," he said. But because of concerns about the unknown objects, he did say draining the water is "an appropriate thing to do."

A surveillance camera captured the incident. Once officials contacted the group, and after the 21-year-old was told he urinated in drinking water and his actions were disrespectful, he reportedly said, "I didn't mean to show disrespect. I thought this was a sewage treatment plant."

Police did not cite the man or his friends, though video and reports by police and the Water Bureau will be submitted to the Multnomah County district attorney's office for possible criminal charges. The Oregonian is not naming the 21-year-old because he was not arrested or charged with a crime.

"It'll kind of depend on what the surveillance video shows," said Sgt. Pete Simpson, a police spokesman. "He's not out of the water yet."

Draining the reservoir will take several days, keeping it offline until perhaps late next week. Shaff originally said dumping the water would cost about $600,000 in lost revenue but he later clarified that his math was way off and that the water would have sold for a retail price of almost $28,500. Disposal fees are expected to reach about $7,600.

Shaff noted that Portland's Bull Run water supply contains billions of gallons of water, and it makes sense to clean the reservoir.

And it goes back to the idea of drinking pee. "Answer the question," he said. "It has nothing to do with 'scientifically.'